Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has recently stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale. Recently, J-G. Zhu et al. described another spintronic device called a spin transfer oscillator in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magnetics, Vol. 44, No. 1, pp. 125-131 (2008) where a spin transfer momentum effect is relied upon to enable recording at a head field significantly below the medium coercivity in a perpendicular recording geometry.
Both MRAM and STT-MRAM may have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers typically referred to as a pinned layer and free layer are separated by a thin non-magnetic dielectric layer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode in a MRAM device. In another aspect, a MTJ element in a read head sensor may be based on a giant magnetoresistance (GMR) effect that relates to a spin valve structure where a reference layer and free layer are separated by a metal spacer. In sensor structures, the MTJ is formed between two shields and there is a hard bias layer adjacent to the MTJ element to provide longitudinal biasing for stabilizing the free layer magnetization.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement and have a low switching current density but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and other spintronic devices. Theoretical expressions predict that perpendicular magnetic devices have the potential to achieve a switching current lower than that of in-plane magnetic devices with the same magnetic anisotropy field according to S. Mangin et al. in Nat. Mater. 5, 210 (2006).
When the size of a memory cell is reduced, much larger magnetic anisotropy is required because the thermal stability factor is proportional to the volume of the memory cell. Generally, PMA materials have magnetic anisotropy larger than that of conventional in-plane soft magnetic materials such as NiFe or CoFeB. Thus, magnetic devices with PMA are advantageous for achieving a low switching current and high thermal stability. Several groups have studied spin transfer switching in GMR multilayers with perpendicular magnetic anisotropy and reported their results including the aforementioned S. Mangin publication as well as H. Meng et al. in “Low critical current for spin transfer in magnetic tunnel junctions”, J. Appl. Phys. 99, 08G519 (2006), X. Jiang et al. in “Temperature dependence of current-induced magnetization switching in spin valves with a ferromagnetic CoGd free layer”, Phys. Rev. Lett. 97, 217202 (2006), T. Seki et al. in “Spin-polarized current-induced magnetization reversal in perpendicularly magnetized L10-FePt layers”, Appl. Phys. Lett. 88, 172504 (2006), and S. Mangin et al. in “Reducing the critical current for spin-transfer switching of perpendicularly magnetized nanomagnets”, Appl. Phys. Lett. 94, 012502 (2009). However, in the GMR devices described in the prior art, typical switching current density is above 10 milli-amp/cm2 which is too high for low switching current MRAM. Furthermore, the MR ratio is around 1% which is too small for the readout signal in MRAM. Therefore, improving the spin transfer switching performance in MTJ elements with PMA is extremely important for high performance MRAM applications.
There is a report by M. Nakayama et al. in “Spin transfer switching in TbCoFe/CoFeB/MgO/CoFeB/TbCoFe magnetic tunnel junctions with perpendicular magnetic anisotropy”, J. Appl. Phys. 103, 07A710 (2008) on spin transfer switching in a MTJ employing a TbCoFe PMA structure. However, in a MTJ with a TbCoFe or FePt PMA layer, strenuous annealing conditions are usually required to achieve an acceptably high PMA value. Unfortunately, high temperatures are not so practical for device integration.
CoPt and its alloys such as CoCrPt and CoPt—SiO2 are not desirable as PMA materials in spintronic devices because Pt and Cr are severe spin depolarizing materials and will seriously quench the amplitude of spintronic devices if incorporated in the spinvalve structures. Similarly, Co/Pd, and Co/Ir will not be good PMA materials for spintronic devices because of the severe spin depolarizing property of Pd and Ir. Furthermore, Co/Pt, Co/Pd, and Co/Ir configurations typically require a very thick and expensive Pt, Pd, or Ir as a seed layer. Au is associated with high cost and easy interdiffusion to adjacent layers which makes a Co/Au multilayer for PMA purposes less practical.
PMA materials have been considered for microwave assisted magnetic recording (MAMR) as described by J-G. Zhu et al. in “Microwave Assisted Magnetic Recording”, IEEE Trans. on Magn., Vol. 44, No. 1, pp. 125-131 (2008). A mechanism is proposed for recording at a head field significantly below the medium coercivity in a perpendicular recording geometry. FIG. 1 is taken from the aforementioned reference and shows an ac field assisted perpendicular head design. The upper caption 19 represents a perpendicular spin torque driven oscillator for generating a localized ac field in a microwave frequency regime and includes a bottom electrode 11a, top electrode 11b, perpendicular magnetized reference layer 12 (spin injection layer), metallic spacer 13, and oscillating stack 14. Oscillator stack 14 is made of a field generation layer 14a and a layer with perpendicular anisotropy 14b having an easy axis 14c. The ac field generator in the upper caption 19 is rotated 90 degrees with respect to the lower part of the drawing where the device is positioned between a write pole 17 and a trailing shield 18. The writer moves across the surface of a magnetic media 16 that has a soft underlayer 15. The reference layer 12 provides for spin polarization of injected current (I). Layers 14a, 14b are ferromagnetically exchanged coupled. Improved materials for the reference layer and oscillator stack are needed as this technology matures.
In other prior art references, U.S. Patent Application Pub. 2008/0170329 discloses a seed layer selected from alloys of Cu or Ta and Ti, and also describes a heat treatment to improve underlayer smoothness in a perpendicular magnetic recording medium.
U.S. Pat. No. 7,128,987 describes a perpendicular magnetic medium with each laminated layer consisting of a composite wherein a discontinuous magnetic phase is surrounded by a non-magnetic phase. Fabrication methods may involve a heat treatment or an unspecified surface treatment of the composite layer.
In U.S. Pat. No. 7,175,925, a seed layer comprised of Ta or Cu is used between a magnetically soft underlayer and a crystalline interlayer in a perpendicular magnetic recording medium.
U.S. Pat. No. 7,279,240 teaches a seed layer of Ti or Cu for a laminated perpendicular magnetic recording medium.
However, none of the aforementioned references suggest a high performance, low cost PMA alternative to the commonly used (Co/Pt)Y and (Co/Pd)Y multilayer stacks in the prior art. A low cost multilayer with high PMA and high Hc is needed to enable PMA materials to be more widely accepted in a variety of magnetic device applications.